Optical fibres of this type (which may generally be referred to as micro-structured fibres) have been described in a number of references, such as WO 99/64903 and WO 99/64904 describing such fibres having claddings defining Photonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, and Monro et al. (see Optics Letters, Vol 25 (4), p.206, February 2000) defining fibres where the light is transmitted using modified Total Internal Reflection (TIR).
Optical fibres and integrated optical wave guides are today applied in a wide range of applications within the areas such as optical communications, sensor technology, spectroscopy, and medicine. The performance of most such applications is affected by or dependent upon the polarization of the light travelling through the fibre. Many systems also suffer from polarization-dependent losses or polarization-dependent propagation properties that can affect system performance. Design of polarization controlling devices in fibre optical systems is therefore vital. The present invention includes design of novel types of polarization controlling optical fibres, for which the polanzation of light coupled to one end of fibre can be preserved at the output endxe2x80x94so-called polarization maintaining fibres.
Light propagating through an optical fibre can be classified as linearly polarised, elliptically polarised and circularly polarised. If the optical fibre has a deviation away from a perfect circular symmetric design (as is the case for optical fibres in practice), this polarization will generally cause the light to split into separate polarization states (two states for the fundamental mode of optical fibres), which will travel at different speeds along the fibre.
For certain applications it is desirable to enhanced such deviationsxe2x80x94and create high-birefringent fibresxe2x80x94as it is e.g., the case for polarization-maintaining fibres (see Agrawal, xe2x80x9cNon-linear fiber opticsxe2x80x9d,Academic Press, Second ed., 1995). For other applications, however, even small deviations may be strongly degrading to the system performance. In particular, for high-speed, long-distance optical communication systems (operating at bit-rates of 10 GHz and beyond), an increased attention is now being paid to polarization effects. For such fast systems even very low degrees of birefringence ( less than 10xe2x88x927) occurring in conventional fibres may cause a crucial dispersion differences between the two polarization states, thereby setting upper limits for transmission speed. To explore the potential of PBG-fibres, it is, therefore, important to predict, understand and tailor the polarization effects, which may result in optical fibres.
The present inventors have realised how to provide a significantly higher degree of freedom for tailoring the polarization properties of micro-structured fibres compared to traditional fibres The present inventors have realised that, in fact, the structure of the cladding elements may act in defining the polarization of light transmitted by the fibre. This type of fibre is realised by providing cladding elements, which are not symmetrical around a centre in the core of the fibre. In this manner, the extent of the cladding elements will differ around radial directions seen from the core(s).
In a first aspect, the invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre having:
a core region extending along the longitudinal direction,
a cladding region extending along the longitudinal direction, said cladding region comprising at least 3 primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, each primary element having a refractive index being different from a refractive index of any material adjacent to the primary element,
each primary element having a shape which, in a cross-section perpendicular to the longitudinal direction, deviates from a circular shape and having parts extending outside a circle having the same area as that of the primary element and having its centre at a centre of the primary element, at least one extending part of each primary element extending at least substantially in a predetermined direction.
A centre of the cross-section of the primary element may be the centre of gravity or the position of the centre of the circle, where the smallest possible area of the primary element extends outside the circle.
Thus, the shape, such as an oblong shape, deviates from a circle having the same area-and consequently parts thereof extend outside the circle.
By applying such non-circular shapes of primary elements, the modal extent of the guided modes of the fibres may be adjusted, so that specific properties may be obtained. Such properties may be related to polarization preserving properties and/or dispersion properties.
These properties may be obtained even if primary elements having different cross-sectional shapes, such as oval shapes and rectangular shapesxe2x80x94as long as the longitudinal axes thereof point in at least substantially the same direction. This is also independent of the relative sizes of the shapes.
In the cross-section, two primary elements may be positioned in a manner so that two lines, each intersecting a centre of a respective of the two primary elements, and both intersecting a centre of the core region, form an angle of at the most 120xc2x0, such as in the interval of 10xc2x0-120xc2x0, such as 15xc2x0-115xc2x0, such as 30xc2x0-110xc2x0, such as 40xc2x0-105xc2x0, such as 50xc2x0-100xc2x0, such as 70xc2x0-100xc2x0.
Another manner of characterizing this structure is one, wherein, in the cross-section, two primary elements are positioned in a manner so that a first distance between a centre of the core area and a centre of one of the two primary elements is at least 2 times a second distance between a centre of the core area and a centre of the other of the two primary elements. This first distance may more preferably be at least 3 times the second distance, such as at least 5 times the second distance. such as at least 10 times the second distance.
Preferably, the cladding region comprises at least 6, such as at least 8, preferably at least 10, such as at least 15, preferably at least 20, such as at least 30 primary elements.
In the present context, the effective refractive index of the cladding is defined or generated by the refractive index/indices of the primary elements and one or more cladding materials positioned between the primary elements.
In one situation, the primary elements may, in the cross-section, cover at least part of lattice points of an at least essentially 2D-lattice. More preferably, to further increase photonic band gap effects all lattice points of a part of the lattice are covered by the primary elements. Naturally, it is preferred that the lattice points at least substantially coincide with centres of the primary elements.
In this situation, the primary elements may, in the cross-section, be rotation symmetric, where a rotational symmetry of the 2D-lattice is different from the rotational symmetry of the primary elements. This may e.g. be a situation where the rotation symmetry of the primary elements is a 120xc2x0 symmetry (such as of a triangular shape) and the rotation symmetry of the lattice is that of a hexagonal (having a 60xc2x0 symmetry). It should be noted that an element not being rotational symmetric (like the shape of a drop) will have a rotation symmetry of 360xc2x0.
In the present context, the predetermined direction will be a predetermined direction in the plane of the cross-section. This direction is preferably the same for the actual extending part of each primary element. Naturally, each primary element may have a plurality of extending parts extending in different directions. In that situation, preferably also these other extending parts extend in predetermined directionsxe2x80x94one direction for each extending part of a primary element.
One relatively simple manner of defining a cladding is one wherein the at least one extending part of each primary element has at least one axis of symmetry, corresponding axes of symmetry of the at least one extending part of each primary element extending at least substantially in the predetermined direction.
The primary elements may, in the cross-section, have at least substantially the same shape, such as a triangular shape. In this situation, different primary elements may have different dimensionsxe2x80x94such as varying dimensions depending on a distance therefrom to the one or more cores. Alternatively and preferably, the primary elements, in the cross-section, have at least substantially the same dimension(s).
When the shapes of the primary elements, in the cross-section, is at least substantially the same, the primary elements may be orientated in at least substantially the same manner. xe2x80x9cOrientated in at least the same mannerxe2x80x9d will, in the situation where the dimensions are at least substantially the same, mean that, in the cross-section, the areas of the primary elements may be created from that of a single primary element by simple translations thereof.
In the present context, the refractive index of the primary elements is different from that of any material adjacent thereto, meaning that this actual change of index is that providing the structure. This step is not dependent on changes of refractive indices outside the immediate area around the circumference of the primary elements. Naturally, this step may be different for all primary elements, but usually the material adjacent to the primary elements is the same throughout the structurexe2x80x94and so is that of the primary elements, whereby this step will be the same at all circumferences around the primary elements.
Preferably, the refractive index of each primary element is lower than a refractive index of any material adjacent to the primary element.
The material of the cladding region adjacent to the primary elements may be any type of material having a suitable refractive index and being suitable in the production of optical fibres. Presently preferred materials are glass types and polymers.
Each extending part may have an area being larger than 3%, such as larger than 5%, such as larger than 10%, such as larger than 15%, such as larger than 20%, such as larger than 25% of the area of the circle.
Also, at least one extending part may extend a distance of at least 5%, such as at least 10%, such as at least 15%, such as at least 20%, such as at least 30%, such as at least 50%, such as at least 75%, such as at least 100% of the radius of the circle and in a direction away from the centre of the circle.
The primary elements may have, in the cross-section, a shape of a triangle, such as an equilateral trianglexe2x80x94such as having a side length being:
larger than 15%, such as larger than 25%, such as larger than 35%, such as larger than 45%, such as larger than 55%, such as larger than 65%, such as larger than 75%, such as larger than 85% than a smallest distance between centres of two primary elements, and/or
larger than 25 nanometers, such as larger than 50 nanometers, such as larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 2000 nanometers, such as larger than 5000 nanometers, such as larger than 10000 nanometers.
Alternatively, the primary elements may have, in the cross-section, a shape of a rectangle, such as having a side length being:
larger than 15%, such as larger than 25%, such as larger than 35%, such as larger than 45%, such as larger than 55%, such as larger than 65%, such as larger than 75% than a smallest distance between centres of two primary elements, and/or
larger than 25 nanometers, such as larger than 50 nanometers, such as larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 2000 nanometers, such as larger than 5000 nanometers, such as larger than 10000 nanometers
The rectangle may have a shape of a squarexe2x80x94such as having a side length being:
larger than 15%, such as larger than 25%, such as larger than 35%, such as larger than 45%, such as larger than 55%, such as larger than 65%, such as larger than 75% than a smallest distance between centres of two primary elements, and/or
larger than 25 nanometers, such as larger than 50 nanometers, such as larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 2000 nanometers, such as larger than 5000 nanometers, such as larger than 10000 nanometers.
The primary elements may have, in the cross-section, rounded corners. Sharp corners may be difficult to manufacture precisely, whereby the structure may be originally designed having rounded, more easily manufactured corners.
Optionally, the primary elements may have, in the cross-section, an oval shape, such as a shape where at least one of the half axes of the oval shape is:
larger than 15%, such as larger than 25%, such than 35%, such as larger than 45%, such as larger than 55%, such as larger than 65%, such as larger than 75%, such as larger than 85% than a smallest distance between centres of two primary elements, and/or
larger than 25 nanometers, such as larger than 50 nanometers, such as larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 2000 nanometers, such as larger than 5000 nanometers, such as larger than 10000 nanometers.
Depending on the intended use and function of the fibre, the dimensions of the structures of the cladding may vary. Presently, it is preferred that, in the cross-section, a shortest distance between centres of two primary elements is being larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 5000 nanometers, such as larger than 10000 nanometers, such as larger than 20000 nanometers.
As described above, different functions are possible using cladding elements such as air holes or rods. One embodiment is one where at least some of the primary elements are positioned so as to form, in the cross-section, a periodic structure in the fibre. In this manner, at least this part of the cladding may exhibit a PGB.
In this embodiment, the primary elements may be positioned so as to form, in the cross-section, a periodic structure in the fibre. When the primary elements form a two-dimensionally periodic structure in the fibre, a Photonic Band Gap structure may be obtained.
In the present context, xe2x80x9csubstantially two-dimensionally periodicxe2x80x9d will mean that it is desired to have an optimal periodicity, but that the manners of production will often alter this. In the prior art it is seen that when circular air holes (low index areas) are introduced into a preform of glass and subsequently drawn, the holes will obtain a not-circular shape which is a sign of this substantial periodicity.
This periodic structure may be defined by a unit cell and a polygon may be defined:
having centres of primary elements in its vertices,
not enclosing any centres of other primary elements than those having their centres at the vertices of the polygon, and
having an area less than or equal to that of the unit cell,
the polygon being a triangle, a rectangle, a square, or a hexagonal.
In another embodiment, the primary elements may be positioned so as to form, in the cross-section, a non-periodic structure. In this manner, normally the light will be guided by TIR where the effective refractive index (for a given wavelength) of the cladding is defined by a cladding material and the number, size and position of the primary elements and optionally additional elements in the cladding.
The above aspect of the invention relates to a specific cladding structure and comprises no limitations what so ever on the core region.
In fact, the invention should be taken as one relating to these specific cladding regions for use in any type of optical fibre in combination with one or more cores or core regions of any type.
Normally, in the situation of band gap structures, the core is taken as an area of the structure, where the periodicity of the band gap structure is broken. The band gap structure is designed so as to make light transmission impossible, and an altering of the periodicity will, consequently, make light transmission possiblexe2x80x94but only in the core region.
A number of different manners exist for defining a core. In the situation of PGB-fibres, one manner is to replace one or more elements of the periodic structure with other elements with different refractive indices, cross-sectional areas or shapes. Another manner is to have the core also have a periodic structure where only one or more elements are not present.
Preferably, in any situation, the core region would comprise a first additional elongated element extending in the longitudinal direction of the fibre.
An especially preferred first additional element is constituted by air or gas and being defined as a void in the material of the fibre. This void may be inscribed within a circle, said circle having a radius being larger than 100 nanometers, such as larger than 200 nanometers, such as larger than 500 nanometers, such as larger than 1000 nanometers, such as larger than 2000 nanometers, such as larger than 5000 nanometers, such as larger than 10,000 nanometers, such as larger than 20,000 nanometers, such as larger than 50,000 nanometers, such as larger than 100,000 nanometers.
In the situation of a PGB structure, the void may have a cross-sectional area in the cross-section being at least 1, such as at least 2, such as at least 3, such as at least 4, such as at least 5, such as at least 6, such as at least 7, such as at least 8, such as at least 9, such as at least 10 times the cross-sectional area of the unit cell.
Using a void, the light may propagate in and/or around air, which provides a number of advantages both as to transmission loss, dispersion, and when used e.g. as a sensor where gas or liquid may be provided within the void to obtain optimum overlap between the light and the gas or liquid.
In a number of different applications, the additional element or any material adjacent thereto may desirably comprise a dopant (e.g. an active or photosensitive material) or a material showing higher order (non-linear) optical effects.
For communication purposes, higher order (non-linear) effects may be used for e.g. soliton communication or more generally in applications, where non-linear effects are influencing the propagation properties of signals in optical communication systems. This also includes realisation of components for optical signal processing.
For applications for fibre lasers or fibre amplifiers, the dopant may be e.g. a rare-earth dopant adapted to receive pump radiation and amplify radiation travelling in the core region.
Alternatively, the dopant may be a light sensitive dopant, such as Germanium. In that situation, the dopant may be use for e.g. optically writing a grating in the fibre or core region.
For sensor applications, the dopant may be a material responsive to a characteristic of a gas or liquid, which response may be detected optically by light travelling in the core region.
In a number of applications, it is preferred that the core region comprises a second additional elongated element, the first and second additional elements being positioned at a distance where light travelling in one additional element is able to couple to the other additional element.
In one application, one elongated element may be a void holding a liquid or gas which may be too turbid for light to travel through. in that situation, the light may travel in the other element while still coupling with the liquid or gas due to the distance between the elements.
In this situation, one may choose to have the liquid or gas travel only in one or both additional elementsxe2x80x94or even in all elongated voids, such as voids of the cladding structure.
Also, by providing two elements between which the light may couple, a number of optical devices may be provided, such as optical fibre couplers. The optical coupling between core elements or core regions may be designed so as to have a predetermined coupling at one or more defined wavelengths, which further makes a number of optical elements possible.
As indicated above, specific advantages will be obtained also when the second additional element is a void.
In fact, especially when the cladding comprises a periodic structure, the fibre may easily be made to comprise a plurality of core regions.
These core regions may be provided sufficiently close for light travelling in one core region being able to couple to one or more core regions.
Alternatively, the core regions may be positioned spaced apart in order to provide a number of separate wave-guides in a single fibre. In fact, the wave guides may be spaced sufficiently apart to allow the band gap structures surrounding each thereof to be different and e.g. be optimised for different wavelengths or wavelength regimes.
Preferably, the core regions are positioned symmetrically within the, preferably periodic, structure, a period of the core regions being larger than a period of the periodic structure.
Naturally, a fibre of the present type may be used for a number of applications where fibres are already used today.
Thus, in a second aspect, the invention relates to a sensor for sensing or detecting at least one characteristic of a liquid or gas, the sensor comprising:
a length of the optical fibre according to the invention, wherein the core region comprises at least a first additional element, the first element being a void extending along the longitudinal direction of the fibre,
means for providing the liquid or gas into the void of the core region,
means for introducing light into the core region, the light being adapted to interact with the gas or liquid in a manner so that the characteristic of the liquid or gas may be determined,
means for detecting light emitted from the fibre and for determining the characteristic of the liquid or gas.
At present, the characteristic may be absorption, absorbance, the presence of a specific agent or material in the gas or liquid, such as for use as a smoke detector, or any other characteristic sensed by an optical sensing method.
If the gas or liquid has a sufficiently low absorption at the wavelength of the light, the introducing means may be adapted to introduce the light into the first additional element. In that situation, an optimum overlap exists between the light and the liquid or gas.
Alternatively, the core region may comprise a second, elongated element extending in the longitudinal direction of the fibre, where the first and second additional elements are positioned at a distance where light travelling in one additional element is able to couple to the other additional element, and wherein the introducing means are adapted to introduce the light into the second additional element. In that situation, the sensing takes place via the light extending from the second to the first element.
In another type of sensor, the characteristic may not be sensed directly by light. In that situation, it may be desired to expose a suitable material to the characteristic, where the response of that material may be sensed optically, Thus, in this situation, at least part of an inner surface of the first additional element may comprise a layer of a material being adapted to alter in response to the characteristic of the gas or liquid, and wherein the introducing means is adapted to introduce light of a wavelength responsive to the altering of the material.
Naturally, the sensor may additionally comprise means for providing the gas or liquid in the fibre, such as for repeatedly providing gas or liquid therein, such as a gas pump if the sensor is used as a smoke detector.
In a third aspect, the invention relates to a fibre amplifier for amplifying an optical signal, said fibre amplifier comprising:
a length of optical fibre according to the invention, wherein the core region comprises a dopant material along at least part of the length, and
means for providing pump radiation to the dopant material for pumping the dopant material so as to amplify the optical signal.
In some cases, fibre amplifiers will, further comprise means for spectrally separating the amplified optical signal from the pump signal, in order not to have pump radiation travelling in the fibre outside the amplifying region.
Especially for communication purposes, the dopant would comprise rare-earth ions, such as erbium, ytterbium, neodymium, praseodymium, etc.
For additional purposes, such as if it is desired to optically write gratings or other structures in the fibre or core region, or simply for increasing the refractive index of the core region, the dopant may comprise a photosensitive material, such as germanium.
In an fourth aspect, the invention relates to a fibre laser for outputting laser radiation, said fibre laser comprising:
a length of optical fibre according to the invention, wherein the core region comprises a dopant material along at least part of the length,
means for providing pump radiation to the dopant material for pumping the dopant material so as to amplify the optical signal, and
feedback means for selectively feeding back at least part of the amplified optical signal so as to repeatedly pass the amplified optical signal through the length of the optical fibre so as to further amplify the optical signal.
Especially for communication purposes, the dopant comprises rare-earth ions, such as erbium.
Also, the dopant may comprise a photosensitive material, such as germanium, in order to facilitate e.g. the writing of gratings in the fibre or core regionxe2x80x94or for increasing the refractive index of the core region.
The manufacture of the optical fibres according to the invention may be as that of manufacturing standard PBG fibres in that rods may be provided having therein holes having the preferred cross-sections.
In the following, preferred embodiments of optical fibres according to the invention will be described with reference to the drawings wherein: